Suppose you are the copyright holder of a book, and are asking $1000. For years, there are no takers; you have no revenue. I come along and would like to read your book, but at most I'm willing to pay $10. I get a little idea: I advertise that I have this book for sale for $10 a copy, and start taking orders. I collect 99 paid orders. At that point, I have $990 of other people's money. I add $10 of my own to make $1000 and buy the book from you. Without your permission, I make 99 copies, and fill the orders.
Am I a pirate? Or a businessman who provided liquidity at a rationally determined market price, and enabled you to finally sell a copy and at least get a $1000?
Also, what amount did I demonstrate a willingness to pay? Certainly not $1000, right? It looks as if my willingness to pay was limited to the $10 of my own that I pitched in.
If I were to be sued, how should the damages be determined?
According to your "demonstrated willingness" concept for calculating damages, all 100 pirates demonstrated a willingness to pay $10, so the damages were $1000. But that's exactly what you got already. Everyone demonstrated willingness to pay $10, forked it over, and it was passed on to you.
So you see, this willingness to pay concept works very well with nonzero amounts too, in such a way that even pirates who charge money can come out smelling like roses.